1. Field of the Invention
This invention relates to the regeneration and manufacture of activated carbon, and more particularly to the regeneration and manufacture of activated carbon adsorbents in a multiple hearth furnace system.
2. Description of the Prior Art
Heretofore, it was known to recycle the gases between various zones in a multiple hearth furnace. For example, in the Gillette, et al. U.S. Pat. No. 2,616,858 issued Nov. 4, 1952, there is disclosed means for recycling gases from the second to the third heating zone. This patent suggested that the steam found in the recycled gas has a beneficial effect and constituted about 15 to 20% of the gas mixutre introduced to the hearth. However, it will be noted that this patent is directed to the regeneration of bone char, wherein the composite consists of about 90% by weight of porous mineral matter, with about 10% carbon disposed throughout. Because of its porous mineral structure, local overburning of the carbon in the bone char is minimized. Moreover, the bone char has a limited capacity to adsorb organic matter. Consequently, it was of little importance whether in any one pass, the organic material was converted to coke rather than being burned in the regenerative process.
In contrast with this, the present invention is directed to the regeneration or manufacture of granulated activated carbon, wherein the main constituent is carbon, i.e. 45 to 95%, from which must be burned the material adsorbed in the fine pores, or present in the carbonaceous starting material which give the carbon its activity. To this end, the quantity of steam (H.sub.2 O) which is available for reactions with the carbon should be on the order of about six times the quantity of carbon gasified. It is therefore apparent that if recycled gas is used, it must contain sufficient steam to allow for the proper regeneration of the activated carbon and, consequently, cannot be obtained according to the method of the aforesaid Gillette patent.
Prior to the disclosure of the present invention, recycled gases were taken directly from the top portion of the furnace through a recycle fan and passed into a combustion chamber. The combustion chamber exhaust gases then entered the bottom portion of the furnace. This provided added means for control of the temperature and atmosphere. Combustion gases of very low oxygen content are thus evolved without excessively high temperatures. Such a method is disclosed in the Charles F. von Dreusche, Jr. Pat. No. 3,153,633 issued Oct. 20, 1964. In certain installations, it was found that such a recycle system resulted in severe maintenance problems which had their source in the elevated temperatures, which are of the order of from about 600.degree. F. to about 1600.degree. F., for example, and in many cases entrained particulate matter and tars. The temperature itself required special recycle fans which are normally high maintenance items. In some cases, the normal operating conditions of the furnace were upset and the temperature would rise to levels for which the recycle fans, ducts and dampers were not designed, resulting in rapid failure of these parts. In addition, the tars and particulate matter adhered to the duct work and fans. This build-up on the fans caused them to become unbalanced, thereby resulting in excessive wear of the bearings. Because of these inherent problems of the recycle system, it became necessary to inject steam directly into the activation zones of the multiple hearth furnace from an outside source, requiring a boiler. Since the cost of fuel has risen dramatically over the past few years, this is an inherent economic disadvantage.
It will be appreciated that with the advent of stringent air pollution bodes, afterburners and scrubbers are now generally required. As a result, over the past few years most of the newer active carbon regeneration and manufacturing systems have installed wet scrubbers to remove the particulate matter from the exhaust gases.